DAR has been conventionally known as a starting material of polybenzbisoxazole (hereinafter to be referred to as PBO) which can be processed into a highly strong fiber having a high elastic modulus. PBO is synthesized by various methods inclusive of a method using resorcinol as a starting material, as described in Japanese Patent Unexamined Publication Nos. 242604/1995, 136/1990, U.S. Pat. Nos. 5,410,083 and 5,453,542, a method using trichlorobenzene as a starting material, as described in Japanese Patent Application under PCT laid-open under kohyo Nos. 500743/1990 and 502028/1993 and a method using m-dichlorobenzene as a starting material, as described in Japanese Patent Unexamined Publication Nos. 238561/1989 and 24038/1991. Industrialization of DAR has been desired.
On the other hand, DAR has a shortcoming that it is susceptible to oxidation. When left in atmosphere, for example, it is discolored due to oxidation in several days, or several hours when the oxidation proceeds faster. When PBO is obtained by polymerizing such DAR as a starting material, only a polymer discolored to green to purple and having a low polymerization degree is obtained. This in turn requires an inert atmosphere for the storage and handling of DAR, as well as complete drying of DAR. Even under such environment, degradation due to oxidation is inevitable by a long-term storage over several months. Therefore, a formed product (e.g., fiber and film) of PBO having superior property and quality has been unattainable by prior art techniques.
Japanese Patent Examined Publication No. 21166/1994 proposes addition of a reducing oxide of phosphorus or sulfur during polymerization to give a high quality PBO. The addition aims at suppressing DAR's decomposition during polymerization, and does not overcome poor storage stability of DAR.
Likewise, U.S. Pat. No. 5,142,021 proposes addition of tin chloride during polymerization for the same purpose as in Japanese Patent Examined Publication No. 21166/1994.
Japanese Patent Unexamined Publication No. 136/1990 teaches addition of tin chloride during production of DAR in an example. One of the purposes of the addition is to use it as a hydrogenating agent of reactive intermediate, nitro-1,3-benzenediol. The other purpose is to reduce impurity with tin chloride during purification by recrystallization. Thus, tin chloride is not present during the storage of DAR, and the addition does not overcome poor storage stability of DAR.
As mentioned above, synthetic methods of DAR have been actively studied and many techniques have been proposed. However, a stabilizing technique of DAR has not been found and industrial production of DAR has not been realized. It is therefore an object of the present invention to provide DAR having an improved storage stability.
The present invention further aims at providing a stabilizing method of DAR during storage.